In the gas transmission industry, it is necessary to operate large reciprocating type compressors at regular intervals along the pipeline. Both engine driven, and electric motor driven reciprocating compressors are used. Traditionally, the engine driven compressors (operating in the 300-rpm range) incorporate a common frame and crankshaft and are called integral reciprocating compressors. More recently, modem high-speed compressors (500–1000 rpm) using higher efficiency drives are installed which use separate frame and crankshaft and are called separable type units. These separable type units can be driven by engines, or electric motors, with the electric motors either fixed or variable speed.
Pipeline compressors often operate unattended for protracted periods in remote locations and require a host of instrumentation to ensure failsafe operation. Periodically, the efficiency of the compressors are measured to determine the health of the unit, as well as ensure optimum performance of the unit. With the increasing desire to improve both the reliability of the industry installed infrastructure, and to reduce engine emissions, real time monitoring of key operating parameters of the compressor is needed. These parameters include compressor power consumption, as well as internal temperature of key reciprocating components.
To measure efficiency of the compressor it is necessary to determine the work performed by the compressor unit. Prior art techniques have involved the calculation of indicated horsepower through the measurement of pressure in the cylinder head or at the discharge and suction sides of the compressor. Problems exist with such methods, however. Pressure sensors can be expensive, inherently produce signal errors, and have limited durability, resulting in lost time and efficiency during replacement of failed pressure sensors. Accurate measurement of compressor cylinder pressure is hampered by the acoustic distortion introduced by the measurement channel between the cylinder and the installed pressure sensor. This distortion is particularly severe on the modem high-speed compressors. Moreover, measurement of cylinder pressure and the resulting calculated work does not include the frictional costs of the piston riding on the cylinder walls. For large compressors in the range of 2000 HP to 10,000 HP or above, these losses can be significant. Accordingly, there is a need for an apparatus for accurately and continuous measurement of the actual work performed by a compressor.